The present invention relates to a novel process for producing sulfur-containing organosilicon compounds which are useftil as additives for rubber. The present invention also relates to a novel process for producing anhydrous alkali sulfides which are useful as synthetic intermediates of the above-mentioned sulfur-containing organosilicon compounds. The sulfur-containing organosilicon compounds are substances which bind synthetic rubber or natural rubber having unsaturated bonds to a filler such as silica, carbon black or surface-reformed carbon black blended into the rubber.
Sulfur-containing organosilicon compounds are industrially useful compounds which have been known from olden times, and many processes for producing them have been known so far.
As a representative process, there is a process wherein halogeno organic silane represented by the following general formula [III] is allowed to react with sodium tetrasulfide, etc. in a solvent at 70.degree. to 80.degree. C. for several hours to give sulfur-containing organosilicon compounds represented by the following general formula [IV]. EQU (R.sup.1 --O).sub.3 --Si--R.sup.2 --X [III] EQU (R.sup.1 --O).sub.3 --Si--R.sup.2 --S.sub.x --R.sup.2 --Si--(O--R.sup.1).sub.3 [IV]
(In each formula, R.sup.1 is a lower alkyl group, a lower aralkyl group, a lower cycloalkyl group, a lower aryl group, a lower alkoxy group, a lower cycloalkoxy group, an aryloxy group, etc., R.sup.2 is a lower alkylene group, X is a halogen atom, and x is an integral number of 2 to 6.) PA1 (R.sup.1 is a lower alkyl group, a lower alkoxy group, a lower cycloalkoxy group, an aryloxy group, etc., and R.sup.2 is a lower alkylene group) PA1 (wherein R.sup.1 is a monovalent hydrocarbon group having 1 to 5 carbon atoms, R.sup.2 is a divalent hydrocarbon group having 1 to 9 carbon atoms, and X is a halogen atom), with each other, to give sulfur-containing silicon compounds represented by the general formula [II] EQU (R.sup.1 --O).sub.3 --Si--R.sup.2 --S.sub.x --R.sup.2 --Si--(O--R.sup.1).sub.3 [II] PA1 (wherein R.sup.1 and R.sup.2 have the same definitions as in the general formula [I], and x is an integral number of 1 to 8).
Examples of known processes are a process wherein the halogeno organic silane [III] is allowed to react with sodium polysulfide such as sodium tetrasulfide to give the sulfiur-containing organosilicon compound [IV] (German Patent Publication Nos. 2,141,159 and 2,212,239), a process wherein the halogeno organic silane [III] is allowed to react with sodium hydrosulfide and sulfur to give the sulfur-containing organosilicon compound [IV] (Japanese Patent Publication Nos. 26671/1982 and 63879/1992), a process wherein the halogeno organic silane [III] is allowed to react with anhydrous sodium sulfide and sulfur to give the sulfur-containing organosilicon compound [IV] (Japanese Laid-open Patent Publication No. 228588/1995), etc.
Both the above-mentioned halogeno organic silane [III], which is the starting material, and the sulfur-containing organosilicon compound [IV], which is the object substance, are liable to be hydrolyzed in the presence of moisture. Accordingly, it is necessary to control moisture carefully during the reactions in order to obtain the object substance in good yields.
Since sodium polysulfide such as sodium tetrasulfide which is used as the raw material in the above-mentioned prior art, and sodium tetrasulfide which is produced by combination of sodium hydrosulfide or sodium sulfide and sulfur (Inorganic and Theoretical Chemistry Vol. II, Longmans Green and Co., Ltd., (1961), p991) have high water absorption properties, it is not easy to obtain anhydrides thereof. A complex drying step is necessary in order to dehydrate these substances, which lowers the yield of the object substance.
As a process wherein anhydrous sodium polysulfide is not used, U.S. Pat. No. 5,596,116 discloses a process wherein sodium ethylate is allowed to react with sulfur, or metallic sodium is allowed to react with sulfur dispersed into ethanol, and then halogenoalkoxysilane is allowed to react with the reaction products. In this process, however, the yields of the object substances are unclear. If sodium ethylate reacts with sulfur, it is unclear what this ethoxide reacts with and even whether or not the ethoxide reacts. U.S. Pat. No. 5,663,396 discloses a process wherein caustic soda is allowed to react with sulfur in a saturated sodium chloride solution, and halogenoalkoxysilane and a phase-transfer catalyst are added to the product in toluene to give sulfur-containing organosilicon compounds. However, this process is accompanied by many side reactions, and yields are low.
There is a process wherein the same procedure as in the above-mentioned process is performed using ammonia instead of sodium as the alkali source. However, a complex step is necessary since hydrogen sulfide and ammonia gas, which are toxic, are used (Japanese Patent Publication No. 12117/1984).
As a process wherein the halogenoalkoxysilane [III] is not used, there is a process wherein alkoxymercaptosilane represented by the following general formula [V] and sulfur chloride or sulfur are used (German Patent Publication No. 2,141,160 and Japanese Patent Publication No. 26671/1982). A method to obtain the alkoxymercaptosilane [V] in a usual manner is known, wherein alkoxychlorosilane is allowed to react with hydrogen sulfide and an alkali metal or an ammonium salt and thiourea (Angew. Chem. Int. Ed. Engl. Vol. 25, (1986), p239). However, further one more step is necessary in order to obtain the sulfur-containing organosilicon compounds of the present invention, and the process is not economical. EQU (R.sup.1 --O).sub.3 --Si--R.sup.2 --SH [V]
Next, prior arts of processes for producing alkali polysulfides used as raw materials of sulfur-containing organosilicon compounds [IV] are described.
As mentioned above, since the alkali polysulfides have high water absorption properties, it is not easy to obtain anhydrides thereof.
Much energy is required in order to obtain anhydrous sulfides from hydrates of sulfides. In addition, since alkali polysulfides such as sodium disulfide and sodium tetrasulfide are viscous liquids under drying conditions, for example, at high temperatures of 120.degree. to 130.degree. C., it is difficult to treat them (European Patent Publication No. 361,998 and Japanese Laid-open Patent Publication No. 228588/1995).
As another method of obtaining sodium sulfide, there is a process wherein sodium alkoxide is allowed to react with hydrogen sulfide (U.S. Pat. No. 5,466,848 and Japanese Laid-open Patent Publication No. 228588/1995). Good anhydrous sulfide is obtained by this process. On the other hand, since hydrogen sulfide is toxic, it is necessary to treat it carefully, and an extra step is required in order to avoid poisoning.
Furthermore, Inorganic and Theoretical Chemistry, Vol. 2, p981 discloses a process wherein metallic sodium is allowed to react with sulfur in liquid ammonia or xylene. However, the reaction using liquid ammonia is carried out at ultra-low temperatures, and treatment of ammonia is also troublesome. Since the reaction in an aromatic solvent such as xylene proceeds explosively at about 98.degree. C., which is a melting point of the metallic sodium, the reaction is not practical.